All Resources

Pop Bottle Rocket, Part III: Force and Mass

In this activity, students use pop bottle rockets to explore how mass affects the flight distances of toy rockets.

2L pop bottle rockets are an excellent way to demonstrate the effects of forces on objects, and provide many opportunities for observing, predicting, measuring and carrying out experiments while controlling for variables.

Part III in this series of activities demonstrates how mass affects the rockets’ distance travelled (its range).

You can do this activity on its own, or in combination with Part I and/or Part II of the series for a deeper investigation into forces and rocket flight.

Mass, Acceleration and Force
If you did Part II, you will have noticed that adding a small amount of water to the bottle causes it to fly much farther. Why is that? 

Water is heavier than air; it takes more force to throw water downward compared to air. In other words, the action force is greater when water is involved, and so the reaction force (the water pushing the bottle upward) has to be greater, too. 

However, there’s a tradeoff. While the water is whooshing out of the bottle, it’s also accelerating a heavier rocket! From Newton’s Second Law, we know that you need more force to accelerate a heavier object. So, there’s an ideal “medium” amount of water to create lots of thrust without adding too much weight.

Safety Tips:
Flying bottle rockets is a lot of fun, but be sure to take safety precautions seriously:

  • Rockets travel far; do not do this activity indoors.
  • Every launcher should be supervised by an adult.
  • Do not pressurize the rocket past 40 psi (pounds per square inch).
  • When pressurizing and launching the rocket, everyone should stand well away from the launcher. Make sure that all observers know that a rocket is about to be launched–a countdown and safety zone combination work well for this.
  • Watch the rocket’s entire flight to make sure it doesn’t hit anyone as it falls. 
  • Use only paper/cardboard fins, never metal. 
  • Wear eye protection. 

Objectives

  • Explore and demonstrate the effects of balanced/unbalanced forces and action/reaction forces.

  • Apply their understanding of forces and their effect on objects to manipulate the flight of toy rockets.

  • Use the scientific process (make predictions, conduct a fair test, control variables, gather and interpret data) to manipulate the flight distance of a toy rocket.

Materials

  • Per Class:
    a pop bottle rocket launcher – made (see instructions from NASA) or purchased (from an hobby /science supply like Boreal)

    a large field or open area
    a 2L pop bottle
    materials to create fins and nose cones (e.g. scissors, tape, construction paper, foam, foil, cardstock)
    pitchers of water
    measuring cups
    a funnel
    a protractor
    safety goggles
    tape measures or metre sticks
    a Water Volume Data sheet
    pencils

Key Questions

  • Why did the bottle go further with some water inside?
  • Does the launcher work if the bottle is filled completely full of water?
  • What volume of water made the rocket travel furthest?
  • Did your prediction match your experimental result?

What To Do

Preparation

  1. Make or purchase a pop bottle rocket launcher. There are several ways to build a launcher using standard materials from a hardware store. Whichever model you choose, be sure to use pressure rated PVC tubing. Instructions can be found in many online videos, or at: NASA | Water Rockets
  2. Build a pop bottle rocket. Add fins and a nose cone to help it fly along a straighter path. Remember that the pop bottle is upside down when it’s launched, so design it with the opening at the bottom. Be careful not to cut into it— any holes or cracks in the bottle will make it impossible to pressurize.

Activity

  1. Choose 5 different amounts of water to test. Write them down in your Water Volume Data Sheet. Make a prediction relating the amount of water to distance and time travelled.
  2. Head to the launcher and pour the first volume of water into the rocket.
  3. Lift the handle on the launch pad and fit the pop bottle onto the nozzle. Be careful not to spill! Ensure that the bottle is locked in.
  4. Find four volunteers to be the “launcher”, the “pumper”, the “timer”, and the “measurer”. The launcher and pumper put on safety goggles, the timer receives a stopwatch, and the measurer, the metre sticks.
  5. The “pumper” adds 5 big pumps of air into the pop bottle. Be sure not to go past about 40 psi (5–6 bike pumps) as greater pressure can rupture the bottle rocket.
  6. The “launcher” positions the rocket, counts down, and launches the rocket. Hint: Make sure that the launch angle is standard across all your tests. Use a protractor to be sure.
  7. The “timer” times the length of the flight from launch to landing and the “measurer” measures the range of the rocket and records it on the data sheet.
  8. Repeat for each of the water volumes, pumping the launcher to the same psi. Record all of the time and distance results.
  9. Discuss the results.

Extensions

  • Follow up with the activity Pop Bottle Rocket, Part II: Projectile Motion. Experiment with the launch angle to see how far you can get a bottle to go.
  • After doing both Part I, II and III combine your discoveries to come up with experimenst that tests the best combination of launch angle and rocket mass.

Other Resources

Science World Resources | Pop Bottle Rockets I: Action and Reaction

Science World Resources | Pop Bottle Rockets II: Projectile Motion